temperature gradient in certain subsurface levels between 30° and 

 the equator must increase, and beneath this region decrease, when 

 compared with the original conditions for an ocean at rest, since 

 the geostrophic equilibrium condition requires a tilting up of the 

 isotherms toward the equator in the layer of the equatorial (grad- 

 ient) current. The depth of this layer depends on the stratification, 

 and it is indicated by the line H — h-4- in figure 3b. 



VII. The hydrostatic mass compensation 



The condition of a steady state requires that for each column 

 of water, the divergence of the total mass transport M from the sur- 

 face to the bottom or to the level of no motion equals zero. Let 

 M 1 be the mass transport of the drift current (in the layer of 

 frictional influence), and let KL be the mass transport of the 

 gradient current (from the surface to a depth where the current 

 vanishes); then div (M 1 + 1^) = 0, or div M-. = - div Mp for each 

 column. Only the total mass transport, integrated from the surface 

 to the layer of no motion, is non-divergent. But the layer of 

 frictional influence is a divergent layer, and this divergence must 

 be balanced by divergent gradient currents in deeper layers. 



In the case of an ocean with constant, or almost constant 

 potential density from the surface to the bottom, the hydrostatic 

 pressure gradient, as caused by the "wind set up", should practical- 

 ly extend down to the bottom (as in figure 3b near 90° of latitude). 

 Here, a "bottom current" as assumed in Ekman's "elementary current 

 system" would provide the necessary anisobaric mass transport in 

 order to balance the anisobaric mass transport in the layer of 

 frictional influence. However, in a strongly stratified ocean, (as 



*3 



